I solemnly swear I am up to no good...
My Christmas oneshot!

Lily lay alone in the girls dorms on Christmas Eve, all dressed up with nowhere to go. She had decided not to attend Professor Slughorn's Christmas Ball after she heard that Potter was going with Samantha Bell. She didn't know why it bothered her, but it just did. She didn't even like Potter.
"Ugh, get a grip, Lily," she muttered to herself, wiping the tears from her eyes as she sat up. She wandered over to the mirror which was standing beside the Christmas tree (which her friend Alice had decorated with several pictures of Frank Longbottom, much to Lily's displeasure. She found the face of Frank Longbottom staring at her every night before bed rather unnerving. Lily was wearing a silk gown of a meadow-y green colour... Like James' eyes, Lily thought. What are you doing? She quickly reminded herself, and firmly slapped her own face. "Ouch..." She whispered, rubbing her cheek.
"Are you sure that slap wasn't for me? Said a voice behind her.
She spun round, off-balance, and almost toppled into the Frank Longbottom shrine Christmas tree. James Potter was leaning on the doorframe of the girls' dormitory. Lily felt her face turn red with frustration.
"I swear to Dumbledore's fantastically extravagant beard, James Potter, I will tell Filch to fix the bloody enchanted stairway if you keep coming up here and surprising me like that!" Lily shrieked.
James made puppy dog eyes, which made Lily's heart melt inside a bit. "Come on, Lily, if you tell Filch, I wouldn't get to see you so often!" Then he made a disgusted face. "Besides if we let Filch inside the Gryffindor Common Room, it'd smell for weeks."
Lily couldn't help giggling at the statement. James smiled at her. Lily quickly coughed, slightly embarrassed, and put on a straight face. "Shouldn't you be at the Christmas Ball with Samantha?" Lily spat the name with contempt. Ravenclaw know it all... she thought.
"Samantha?" James looked confused. "Samantha who?"
"The Ravenclaw who you took to Slughorn's Christmas Ball!"
James looked even more perplexed. "I didn't take her to the Ball."
"Oh..." Lily felt slightly embarrassed. "Well, which poor girl did you take then?"
"No one."
"Oh."
Lily felt James stare at her, and her cheeks burnt in his gaze. She looked at the floor. Stupid Marlene, she thought, she was the one who told me he was taking her.
"Why do you ask?" James asked her.
"Oh... Well... I, er..." Lily shifted on her feet, feeling slightly awkward.
"Oh yes, that makes me think, now that you remind me..." James' eyes sparkled mischievously. "Do you want to come with me?"
Lily went bright red. "Not in your wildest dreams, Potter! Over my dead body! I'll only say yes when I like you, which means NEVER! You can-"
Before her brain registered anything else, James had grabbed her by the waist, twirled her round and kissed her. Lily just stood there, shell-shocked, staring at him. She was speechless. Her lips tingled from where James had kissed her. "Urm..." Lily mumbled. "Well, I suppose you could take me. That is, if you wanted to."
Lily felt James stare at her. She couldn't bear the silence, so she decided to fill it. "If you don't want to, I mean, that's fine, because I have studying to do, you have studying to do, I mean, that's what 7th year is like, isn't it? And because we're the Heads, we should be studying the most right? And -"
Suddenly, James started laughing. He was just chuckling to himself, and Lily felt very small. "Don't laugh! Do I have something in my hair? Oh Merlin! I think I still have snow in my hair! Has it melted? I mean, it should have -"
"Evans?"
"Yes?"
"Look up."
Lily stopped rambling and looked up. Magical mistletoe was forming above their heads, entwining itself down from the ceiling. James blushed and looked at the floor. "I know a spell to make it go away, if you want. I made it after Padfoot and this random Hufflepuff-"
"James?"
"Yes?"
"Shut up."
Just then, Lily threw her arms round James and kissed him. James wrapped his arms round Lily's shoulders, and they stood there for Merlin knows how long, kissing under the mistletoe. After a while, Lily pulled away, and stared into James' eyes.
"Now, James," she said, staring at him seriously. "Not a word about this to anyone. Or I'll give you a detention."
James shrugged. "I'm booked with detentions until February."
"Then I'll take 100 points off Gryffindor."
"Really? Your own house? Besides, you'd just earn them all back in a week," he pointed out.
Lily scowled. "Ugh, I can't win. Very well then."
James smirked triumphantly. "Don't we have a Christmas Ball to go to?"
Lily smiled back. "Thought you'd never ask."
Then, with a quick peck on his cheek, she said, "Merry Christmas, James."

Merry Christmas!
...Mischief managed.